Security documents can have a thread or strip as a security element which is embedded in the document (German open application 39 06 695). The security element can be comprised of a light-permeable synthetic resin foil which can have an opaque coating extending over the element with openings in the form of applied markings and patterns. In addition, the security element can contain regions of a coloring and/or luminescent substance coextensively provided in the recesses which can distinguish the marks and/or patterns under appropriate light conditions from the security document and from the opaque layer in a color-contrasting manner. As a consequence, the marks, patterns or the like are visible in translumination and can be read visually and/or by machine.
In the security paper field there are, however, also problems in that color-copying devices have increasingly higher quality of compositions of the copies so that, especially with fleeting consideration as is customary in business life, the copies can scarcely be distinguished from the original. Many attempts have been undertaken to develop processes which will make copying of security paper impossible or at least will supply copies which can be distinguished from the original conspicuously. It has thus been proposed (German open application 33 14 670) to coat on at least one part of the surface of a verifiable document, a material which, upon lighting with illumination light, will tend to meet light of a wavelength which is different from the wavelength of the illumination light.
If one places a document thus prepared in the illumination light of a copier and collects emission light with a detector, the latter can generate signals which can be evaluated to interrupt or interdict at least one of the steps required for the duplication of the original document in the copier. The preparation of the document can pose difficulties and the applied material can change or ablate so the security function is no longer fulfilled.